Stephen was born in the County of Blois in middle France; his father, Count Stephen-Henry, died while Stephen was still young, and he was brought up by his mother, Adela. Placed into the court of his uncle, Henry I, Stephen rose in prominence and was granted extensive lands. Stephen married Matilda of Boulogne, inheriting additional estates in Kent and Boulogne that made the couple one of the wealthiest in England. Stephen narrowly escaped drowning with Henry I's son, William Adelin, in the sinking of the White Ship in 1120; William's death left the succession of the English throne open to challenge. When Henry I died in 1135, Stephen quickly crossed the English Channel and with the help of his brother Henry of Blois, a powerful ecclesiastic, took the throne, arguing that the preservation of order across the kingdom took priority over his earlier oaths to support the claim of Henry I's daughter, the Empress Matilda.

The early years of Stephen's reign were largely successful, despite a series of attacks on his possessions in England and Normandy from David I of Scotland, Welsh rebels, and the Empress Matilda's husband, Geoffrey of Anjou. In 1138 the Empress's half-brother Robert of Gloucester rebelled against Stephen, threatening civil war. Together with his close advisor, Waleran de Beaumont, Stephen took firm steps to defend his rule, including arresting a powerful family of bishops. When the Empress and Robert invaded in 1139, however, Stephen was unable to crush the revolt rapidly, which took hold in the south-west of England. Captured at the battle of Lincoln in 1141, Stephen was abandoned by many of his followers and lost control of Normandy. Stephen was freed only after his wife and William of Ypres, one of his military commanders, captured Robert at the Rout of Winchester, but the war dragged on for many years with neither side able to win an advantage.

Stephen became increasingly concerned with ensuring that his son Eustace would inherit his throne after him. The king attempted to convince the Church to agree to crown Eustace to reinforce his claim; Pope Eugene III refused, and Stephen found himself in a sequence of increasingly bitter arguments with his senior clergy. In 1153 the Empress's son, Henry FitzEmpress, invaded England and built an alliance of powerful regional barons to support his claim for the throne. The two armies met at Wallingford, but neither side's barons were keen to fight another pitched battle. Stephen began to examine a negotiated peace, a process hastened by the sudden death of Eustace. Later in the year Stephen and Henry agreed to the Treaty of Winchester, in which Stephen recognised Henry as his heir in exchange for peace, passing over William, Stephen's second son. Stephen died the following year. Modern historians have extensively debated the extent to which Stephen's personality, external events, or the weaknesses in the Norman state contributed to this prolonged period of civil war.